This invention relates to batch mixers, and particularly to mixers for critical materials such as high energy fuels which are potentially explosive. Typical mixers of this character are described in the present assignee's U.S. Pat. No. 3,075,746 and provide off-axis mixer shafts with intermeshing blades thereon extending into a static mixing bowl. In such mixers, the shafts typically orbit in the bowl at the same time they are rotating about their own axes. Moreover, the shafts and operating mechanisms are enclosed by a static enclosure and releasably sealably connected to the jacketed bowl.